During the preovulatory and estradiol-progesterone-induced GnRH-LH surge, a subpopulation of GnRH neurons transiently expresses the transcription factor c-fos, which is a useful marker of cell activation. To further characterize this subpopulation of GnRH neurons, multiple immunohistochemical procedures were applied to visualize GnRH, c-Fos, KA2, GluR5, GluR6, and GluR7 receptor subunits during different phases of the estrogen-progesterone-induced LH surge. The results show that the LH surge begins at 1400 h and peaks at 1600 h before returning to baseline late in the evening. At 1400 h, about 50% of the GnRH neurons contained c-Fos, and this percentage remained high at 65% at 1600 and 2000 h. During the surge, 50% of the c-Fos-positive GnRH neurons contained KA2 receptor subunit protein at 1400 h, 65% of the c-Fos-positive GnRH neurons expressed the KA2 subunit at 1600 h, and 50% of the c-Fos-positive GnRH neurons expressed the KA2 subunit at 2000 h. As KA2 subunits require other kainate-preferring subunits to form functional receptor channels, we examined GnRH neurons for the presence of GluR5, GluR6, and GluR7 messenger RNA (mRNA) and protein. The results show that the KA2-containing GnRH neurons also contain GluR5 receptor subunit mRNA and protein, and that these GnRH neurons are c-Fos positive during the steroid-induced LH surge.